Fiber Bragg grating sensors (FBG sensors) have attracted considerable attention in sensing temperature and strain on an optical fiber due to the variation in the spectral response of the grating as a result of strain and temperature on the grating structure. FBG sensors offer important advantages such as electrically passive operation, immunity to electro-magnetic interference (EMI), high sensitivity and multiplexing capabilities. Fiber gratings are simple, intrinsic sensing elements which traditionally have been UV photo-inscribed into photosensitive Ge-doped silica optical fiber. Each FBG sensor has a characteristic retro-reflective Bragg resonance or Bragg wavelength λBr, which is dependent upon the periodicity of the grating within the fiber and the effective refractive index of the optical fiber. The FBG sensors can then easily be multiplexed in a serial fashion along a length of single mode fiber. When embedded into composite materials, optical fibers with an array of FBG sensors allow for distributed measurements of load, strain, temperature and vibration of the material creating what is commonly referred to as “smart structures” where the health and integrity of the structure is monitored on a real-time basis. The concept of using a Bragg grating as a sensing element was taught by Morey in U.S. Pat. No. 4,996,419.
The main sensing parameter monitored for FBG sensors is the resonant Bragg wavelength λBr of the grating structure. Unfortunately it is often difficult to discriminate between different effects, for example temperature and stress, with a single FBG since the different effects can impact simultaneously on λBr. Often another Bragg grating in a favorable arrangement is used for each of the parameters involved in a particular case, but this procedure will result in a more complicated sensing configuration. For many applications it is desirable to have the capability to measure more than one effect from a single sensor element. For example Udd discloses in U.S. Pat. No. 5,828,059 the dual wavelength birefringent response from a single fiber grating in a birefringent optical fiber that can be used to simultaneously measure temperature and lateral pressure.
Typically fiber Bragg gratings are generated by exposing the UV-photosensitive core of a germanium doped silica core optical fiber to a spatially modulated UV laser beam in order to create permanent refractive index changes in the fiber core. Such a spatially modulated UV beam can be created by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al.
A limitation of the prior-art UV-induced fiber Bragg gratings, especially for high temperature sensor applications is that operation of the sensor at elevated temperatures results in the erasure or annealing of the UV-induced color centers which are responsible for the induced index change of the grating. In fact, at temperatures approaching the glass transition temperature of the fiber, which for silica is approximately 1200° C., total erasure of the induced index modulation results. The optical fiber also is modified at such high temperatures making it brittle with diffusion of the core material into the cladding. The fiber can easily be deformed by its own weight.
The prior art FBG sensors suffer from serious limitations when measurement of displacement, temperature, strain and pressure are required at high temperatures. The materials used to fabricate the FBG sensing element deform or melt. The softening or glass transition temperature of silica optical fibers is typically 1200° C. At temperatures equal to or above this, silica optical fibers are not useful, even if they are coated with materials that melt at higher temperatures.
One approach to fiber-based measurements at high temperatures is to use sensor elements fabricated in fibers made of sapphire. Because sapphire has a very high glass transition temperature (˜2030° C.), a sensor fabricated in this fiber can be operated in high temperature environments. Currently, sapphire fiber waveguides are only made in the form of rods with diameters as low as 50 μm. These rods lack a cladding or a coating material similar to conventional optical fibers. The large diameter of the sapphire fiber does not support single mode propagation at typical wavelengths used for FBG sensors in silica fiber thus does not allow the implementation of the FBG sensor as described previously. Murphy et al. in U.S. Pat. No. 5,381,229 have taught a technique for the fabrication of a sapphire optical fiber interferometric sensor based on the fabrication of a Fabry-Perot etalon on the tip of the sapphire fiber. Although this device is effective as a point sensor, is relies on the monitoring of the broadband interference fringe pattern generated by the Fabry-Perot etalon and therefore is extremely difficult to address in a wavelength-division or time-division multiplexing fashion. This makes the Fabry-Perot based fiber sensor inappropriate for distributed sensor arrays.
In another approach, Dils discloses in U.S. Pat. No. 4,750,139 a blackbody radiation sensing optical fiber thermometer system that employs a sapphire rod terminated in a black body tip composed of iridium sputtered onto the end of the rod. As with the Fabry-Perot based fiber sensor approach of Murphy et al., the sensor by Dils is effective only as a point sensor.
Mihailov et al. in U.S. Pat. Nos. 6,993,221 and 7,031,571, incorporated herein by reference, disclose techniques for fabrication of Bragg grating structures in optical media such as optical fibers and waveguides with an ultrafast (<500 ps) laser source and a phase mask using a direct writing technique. The resultant grating structures have high induced-index modulations (>1×10−3). Since the refractive index change need not be dependent on the dopant in the core or cladding of the optical fiber or waveguide, refractive index changes can be induced in both regions of the waveguide. Mihailov et al. in U.S. Pat. No. 7,379,643 incorporated herein by reference, disclose how this technique of Bragg grating inscription using ultrafast laser pulses can be used to inscribe a Bragg grating sensor in optical waveguides such as sapphire that have much higher melting temperatures than silica. Mihailov et al. also teach in U.S. Pat. No. 7,379,643 how optical fiber tapers can be used to excite low order or fundamental modes of the multimode sapphire waveguides in order to improve the spectral response from the sapphire fiber Bragg grating for sensing applications facilitating the use of these gratings in distributed sensor arrays.
Recently Busch et al. disclosed in their paper, “Inscription and characterization of Bragg gratings in single-crystal sapphire optical fibres for high-temperature sensor applications,” in the journal Measurement Science and Technology, vol. 20, no. 11, pp. 115301, 2009, incorporated herein by reference, that at high temperatures of 1745° C., the blackbody radiation produces a strong background spectrum with the sapphire optical fiber, which reduces the signal to noise ratio making the detection of a multimode Bragg resonance more difficult.
Recently Grobnic et al. disclosed in their paper entitled: “Multiparameter sensor based on single high-order fiber Bragg grating made with IR-femtosecond radiation in single mode fibers,” in the journal IEEE Sensors, vol. 8, no. 7, pp. 1223-1228 (2008), incorporated herein by reference that high order Bragg gratings written in single mode optical fiber produce Bragg reflections or resonances that differ in wavelength but also respond differently to parameters such as strain and temperature. The Bragg resonance is defined bymλBr=2neffΛ  (1)
where neff is the effective refractive index seen by the resonating guided core mode, Λ is the period of the grating within the waveguide and in is the diffracted order number of the Bragg resonance λBr. As the wavelength resonances generated by the individual diffracted orders vary differently when subjected to environmental parameters such as temperature and strain, a single grating element that produces these multiple resonances can be used to decouple simultaneously strain and temperature effects from the shift in Bragg wavelength. The results presented are limited only to single mode fibers.
It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of a high temperature FBG sensor that can simultaneously measure more than one effect such as temperature and strain.
It is a further object of this invention to provide a method for either increasing either the coupling of black body thermal radiation into a sapphire optical fiber by inscription of a sapphire FBG or having that grating element act as an emissivity element within the fiber. Advantageously, by promotion of the black body radiation level, a sensor system can be made that does not require an optical source to probe the grating sensor as in traditional FBG sensor arrays. By having improved signal to noise ratios through the excitation of fundamental or low order modes of the multimode sapphire fiber as taught by Mihailov et al. in U.S. Pat. No. 7,379,643 a dual stress/temperature sensor is realized. By monitoring the signal level of thermal blackbody radiation as a temperature reference, the portion of the wavelength shift of the Bragg grating in the sapphire fiber that is dependent on temperature can be decoupled from the strain. This device would be useful for monitoring strains in high temperature environments.